112 THE FEOG. 



6. Comparison of the Early Stages in the Development of the 

 Frog with those of Amphioxus. 



The frog's egg is more than 6,000 times the balk of that of 

 Amphioxus : this large size is due mainly to the much greater 

 amount of food-yolk present in the frog's egg, and it is chiefly 

 owing to this food-yolk that the development of the two forms 

 is so different. In the earliest stages the differences are less 

 marked than in the succeeding ones. The first two segmenta- 

 tion clefts divide the frog's egg in the same way as they do that 

 of Amphioxus ; the third cleft is in both cases a horizontal one, 

 but while in Amphioxus it is nearly equatorial, in the frog ' it 

 lies much nearer the upper pole. The stage shown for the frog 

 in Pig. 49 corresponds fairly closely, in essential respects, with 

 the blastula stage of Amphioxus ; but from this point the 

 development of the two forms becomes widely different. 



T here is no stage in the frog which exactly corres^prmrlg t.r> 

 the gastrula stage in Amphioxus ; for at the stage shown in 

 Fig. 52, which most nearly approaches to this, both epiblast and 

 hypoblast are already three or more cells thick, instead of being, 

 as in Amphioxus, single layers of cells. Moreover, t he primitiv e 

 di gestive cavitv of th e frog (Fig. 52, t) i s formed, not bv inva gi- 

 n ation,^ as in Amphioxus, but by a process of splitting, or separa- 

 tion, among the yolk-cells occupying the interior of bhe embryo. 

 The history of development in some allied animals, notably in 

 the newt, suggests that the process of splitting is a secondary 

 modification, which has arisen in consequence of the hindrance 

 offered by the large mass of yolk-cells to the occurrence of 

 invagination. 



Th e early establishment of the two-layered condition of the 

 epiblast is anot her point in which the frog presents a mod ified 

 and specialised condit-.ion : in the corresponding stages of the 

 newt the epiblast consists, as in Amphioxus, of a single layer of 

 cells. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 



It will be convenient from this point to deal with the several 

 systems one by one, following each up to its condition in the 

 adult. The order in which the systems are taken is chiefly a 

 matter of convenience, but for several reasons the nervous 



